804

THE COLLIERY GUARDIAN.

October 27, 1916.

Coal Conciliation Board for England and North Wales,
admitted that normally there was usually an upward
curve in the percentage of absenteeism between July
and September, and if they met again in two months
he hoped they would not attribute the decline in absen-
teeism entirely to their own efforts.

It is understood that no resolution was submitted to
the meeting, and no action was taken beyond the recep-
tion of the figures as to absenteeism, and a very general
discussion, the attitude being one of waiting to see what
would happen at Wednesday’s National Conference.

In the afternoon the men’s leaders held a meeting,
and Mr. Smillie made the following statement :—“ The
executive committee expressed themselves as satisfied
with the joint meeting they had held with the mine
owners. It was clear that the progress which had
been expected with regard to absenteeism had not been
fully realised, but the executive felt sure that a con-
siderable improvement could yet be made in that
direction.”

EXTRACTION OF COAL BY
SOLVENTS.

In a communication to the Kaiser Wilhelm Institute
for Uoal Investigation, F. Fischer and W. Gluud give a
summary of the work previously done by other investi-
gators, and also an account of their own experiments
with the use of benzol and sulphurous acid as solvents.

Coal being imperfectly soluble in any known reagent,
it is found necessary to subject it to chemical treatment
in order to obtain definable and soluble modification
products, but, whilst such transformation products can
be formed by nitration, sulphonation, etc., the yield is
either so small or the products themselves are in such
an advanced stage of degradation that they do not
afford any reliable information as to the original nature
of the coal substance. Where higher yields or less
degraded products have been obtained, they have
generally consisted of inseparable mixtures and solutions
—chiefly of a colloidal nature—as to preclude further
analysis. In this direction there is, however, a wide
field still open for research of a highly profitable
character, and the authors already have in hand experi-
ments on the influence of ozone on coal, from which
early results are expected.

Another line of investigation is by distillation, and
this has recently been brought considerably nearer
perfection by working in vacuo, though it must be
admitted that the high temperatures involved militate
against the applicability of this method to coal. More-
over, only a small portion of the coal is rendered
available for further examination, the bulk of the mass
being in the form of coke, which certainly has nothing
in common with the original coke substance. Neither
the study of coal tar nor that of coke seems likely to
bring us any nearer a solution of the coal problem.

A third method of ascertaining the constituents of
coal is that of extraction with solvents, but even with
this one must be content with obtaining only a fractional
part of the material in an unaltered state. However,
the results obtained by the authors are forming
the basis of further investigation, which may
probably turn out to be of technical interest.
In any event, the extraction method is, at present,
decidedly superior to either of the others, since it
enables substances to be recovered without alteration.
This applies not merely to the constituents extracted in
the soluble form—which may perhaps be regarded as
tar formers—but also to the insoluble residue, which
may be considered to consist of the coke formers.
There is therefore reason to hope that extraction can
effect a fairly clean separation of coal into its two main
groups of components, the tar formers and the coke
formers, and thus divide the coal problem into two for
the purpose of investigating the extractives (tar
formers) on the one hand and the residue (coke formers
or hydrogen formers—in gas making) on the other.

This view of the composition of coal, which is
intended solely as a working hypothesis, is justified foi*
numerous reasons, and has also been applied in a
similar manner by other workers. In the first place,
the experience gained in coal distillation speaks in
favour of it, and Wheeler’s intimate investigation of
coal distillation has led him to adopt the same view.
On the other hand, the hypothesis that coal consists of
plant remains of divergent character and origin affords
a lucid explanation of the occurrence of a variety of
types of substances in the mass of the coal, and also
some indication of how they may be grouped,
chemically. One may, for example, regard the extrac-
tives as products of the modification of pre-existing fats,
waxes, resins and so forth, whilst the predominant bulk
might be directly considered as modification products
of substances of the nature of cellulose and lignin. At
the same time a ready explanation is afforded of the
preponderance of the one constituent on the one hand,
owing to divergent proportional mixtures in the fossil
plants, and on the other in consequence of the varying
stability of the transformation products.

Starting with this view as a basis, and according
it, for the time being, no higher value than that of a
working hypothesis, it will at once be evident what is to
be expected from the extraction method of coal investi-
gation, and what means are to be used. The most,
therefore, that extraction can be hoped to furnish is to
recover from the coal the former fats, waxes, resins,
&c., or the corresponding modification products, that is
to say, only about 10 per cent, of the bulk; and the
fact that this maximum yield has not hitherto been
obtainable led the authors to carry on the experiments
described later on.

It is also a necessary condition of all extraction
processes that they leave the materials chemically
unchanged, in order that their divergent solubility, in
the form in which they actually exist in the coal, as a

result of their difference of origin, may be utilised.
This condition involves the necessity of a critical
sifting of the solvents used, so as to exclude such as do
not appear suitable for the purpose in view.

Earlier Work.

Pyridin as Solvent.—The surprising powers of pyridin
as a solvent have led to its employment by various
investigators, of whom Bedson ap} ears to have been
the first (1899). The results obtained were set out by
him, some years later, in the Journal of the Society of
Chemical Industry (1908, p. 147). The usual method
of extraction consisted in treating the finely powdered
coal for several hours in a Soxhlet apparatus, a treat-
ment which furnishes dark brown solutions (sometimes
with green fluorescence) containing extractives ranging
in amount from zero in the case of anthracite to 33 per
cent, with good gas coal. Theie does not, however,
seem to be any correlation between the amount of
extract and the content of volatile substances, inferior
gas coals yielding as much extract as g- od ones ; whilst,
in the case of several samples of cannel coal, the
extract varied between 7 and 20 per cent. The caking
properties of the coal seem to be more or less impaired
by extraction with pyridin; and Bedson admits the
impossibility of drawing any reliable conclusions as to
the coal from the amount of extract obtained. More-
over, there can be no doubt that the “solvent” action
of pyridin on coal is accompanied by chemical de-
composition.

Owing to the strong intumescence produced when
the coal comes in contact with pyridin in the Soxhlet
apparatus, Wahl proposed to add an inert substance
(salt or potassium sulphate), which could afterwards be
readily leached out with water. The extract thus
obtained, when precipitated with hot acid, washed
and dried, is an amorphous powder insoluble in
water, alkalies and acids (except fuming nitric acid,
which has an oxidising effect). It is, however, soluble
in organic solvents, and furnishes analytical data
corresponding closely—even in respect of nitrogen
content—to those of the original coal. The yield
varies between 0 and 25 per cent., and it is a remarkable
feature that, even after the removal of 20 per cent, of
extract, the percentage of volatile substances in the coal
is very little lower than before treatment.

Harger, in 1914, confirmed the report of Dennstedt
and Biinz that the pyrid n extract contains sulphur and
nitrogen, as well as carbon, hydrogen and oxygen ; and
that the analysis of the extract agrees with that of the
original coal, the amount of nitrogen found being even
larger. He introduced a modification of the method by
extracting at high temperatures in closed tubes, and,
by heating at 160-180 degs. Cent, for 24 hours, obtained
40 5 per cent, of extract from a coal which, under the
ordinary method, yielded only 22 5 per cent, in 14 days.
He, too, came to the conclusion that the action of
pyridin on coal is not a case of ordinary solution.
Other work in the same direction has been done by
Donath, Rau, Vignon, Niibling and Wanner, the last
two observers attributing the spontaneous ignition of
coal to the constituents extracted by pyridin.

The conclusion to be drawn from the experiments
with pyridin is that, although this solvent may extract
more from coal than other reagents are able to do, it
does not clearly differentiate between the soluble and
insoluble constituents of coal in the way an extraction
method ought to do, the issue being confused by the
formation of salts and stable soluble transformation
compounds, which increase the complexity of the
extract. This view was confirmed by Lewes, who, whilst
admitting the solvent properties of pyridin, complained
of its troublesome tendency to attach itself to other
molecules, and thus becoming difficult to eliminate
without resort to such drastic measures that the nature
of the dissolved substances is altered.

Anilin and Quinolin as Solvents.—The same objections
attach to the use of anilin and quinolin as solvents for
coal. Vignon, who recently investigated the action of
anilin, reported that the empirical composition of the
extracts is the same as that of the original coal (within
the limits of experimental error). From three Montram-
bert coals he obtained the following data:—(1) Bitu-
minous gas coal, 23*4 per cent, of extract (26 8 per cent,
referred to ash-free coal); (2) semi-bituminous coal. 6'58
per cent. (7’2 per cent.); lean coal, 1'56 per cent. (1*8 per
cent.). To obtain this result the coal was boiled for an
hour with anilin under a reflux condenser (at 182 degs.
Cent.), and the yield determined from the loss in weight
after the elimination of the anilin from the coal by
means of dilute hydrochloric acid.

According to Vignon, quinolin is superior to pyridin
or anilin, the loss in weight sustained by the coal after
boiling for an hour at 238 degs Cent, being 4" *3 per cent.;
but it is uncertain whether this increased yield is not
mainly due to the higher temperature.

Action of Alcoholic Potash on Coal.—Although alco-
holic caustic potash gives a large yield of extract from
coal, its use is attended by the same drawbacks as
pyridin, and therefore the authors passed it over in their
investigations.

Phenol.—The same criticism applies also to phenol,
which requires a high temperature, and is also chemically
active, although it was in favour for a long time on
account f the high yield of extract produced.

Inert Organic Solvents.—If recourse be had to solvents
that are quite inert as regards action on the coal con-
stituents, it is found that none of them gives more than
insignificant quantities of extract. Experiments with
alcohol, ether, amyl acetate, carbon disulphide, toluene
and chloroform have shown that these are, for the most
part, applicable only at comparatively low temperatures,
and furnish poor results.

Benzol.—This has been found the most suitable solvent
of all, being quite inert, chemically speaking, and able
to stand high temperatures without decomposing and
producing complications, in addition to which it is

easily eliminated completely from the extract. Unfor-
tunately, however, the yield of extract is very low, for
which reason Pictet tried distillation in vacuo in the
hope of obtaining larger results. Nevertheless, the
method has the drawback that it affords information
about only a very minute proportion of the coal (about
one-thousandth part of the total coal substance) leaving
the other 999 parts still unknown. Woiking with the
same solvent, Watson Smith obtained yields of
0*5 to 1 per cent, from a number of cannel coa!s,
crushed small and extracted for 14 days in a Soxhlet
apparatus, until the benzol came away quite colourless.
Hau obtained 1 to 1A per cent, of extract from
Westphalian coal by the use of a higher temperature
and increasing the pressure to 14 atmospheres.

The Authors’ Experiments.

Benzol Extraction under Pressure.—Attempts were
made in the first place to increase the yield of extract,
in order to render a larger proportion of the coal
accessible to examination; and on the other hand to
obtain a clean separation between the soluble and
insoluble constituents. In these experiments they
succeeded in improving the yield to a not inconsiderable
extent, and in fact to a level equal, to any of the
distillation methods. The method of extraction with
benzol thus acquires far mote importance than hereto-
fore, and although the products are quite different from
those obtained by distillation, they may be equal in
value, if not superior to the tar constituents on which
the dyestuff industry has been built up. In any event,
a far broader basis for scientific investigation is afforded
than by the insignificant yields previously furnished by
benzol extraction. .

The three principal types of coal: pit coal, brown
coal and bituminous coal (Germancannel), were treated,
the temperature and pressure being raised beyond the
ordinary laboratory limits and approximating to the
critical constants of the solvent (about 288 degs. Cent,
and 50 atmospheres).

The results obtained may be briefly summarised as
follows:—Pit coal gave a yield of over 6| per cent,
of extract, as compared with 01—0*15 per cent, at
80 degs. Cent.; brown coal yielded 25 per cent., in place
of 11 per cent, in the Soxhlet apparatus ; and the cannel
coal furnished 4 per cent, against a little over I per
cent, at 80 degs. Cent.

On the scientific side the increase of the yield from
the pit coal is the most interesting, since in this case
the extraction method is equivalent to the various
distillation methods, which, too, furnish only 4 to 6 per
cent, of tar. From the technical standpoint the improve-
ment of yield in the case of the brown coal is probably
of importance, because extraction with benzol is already
practised in this connection.

In concordance with the reports of W heeler, who also
observed that the incipient decomposition of coal does
not occur until the temperature exceeds 270 deg. C., the
authors failed to discover any liberation of gas on
opening the cooled bomb after extractions up to that
temperature. Nor did they find any smell of sulphur
compounds, which is usually the first indication of
incipient decomposition, in extracting the pit coal and
cannel. On the other hand, with brown coal, consider-
able quantities of sulphuretted hydrogen were evolved
during the first experiment.

The authors believe, therefore, that no important
decomposition occurs with pit coal or cannel under the
conditions of their experiments, and that a portion of
the substances originally present in the coal is also
recovered in the extract. They are also aware that the
increased temperature and pressure are probably not
without influence on certain of the soluble constituents
of coal, and may, in particular, facilitate any tendency
toward polymerisation. Nevertheless, they are of
opinion that this extraction method is at least equal to
vacuum distillation, even for the scientific investigation
of pit coal, especially since the temperature they
employed was more than 150 degs. Cent, lower than
that used in vacuum distillation.

If it be desired to push the extraction as far as
possible, without consideration of possible decomposition
of the coal, naphthalene seems to be a very suitable
solvent, since ib does not develop such a powerful
pressure, whilst, like benzol, it withstands high
temperatures. On the other hand, it has the drawback
that it cannot be so easily expelled from the extract.

Up to the present there appears to be only one report
on the extraction of coal with benzol at high tempera-
ture and pressure, namely that of Rau (on Westphalian
coal), who, working at 209 degs. Cent, and 14 atm< spheres
pressure, obtained only 1 to per cent, of extract,
whereas the authors obtained over 6 per cent. Owing
to the absence of further particulars in Rau’s report,
there is no possibility of making any comparison. In
the authors’ experience the coal is far from being
exhausted by a single extraction, even at such a high
temperature, and when the extract is not in the state of
a saturated solution. Possibly Rau refern d only to a
single extraction, whereas the authors repeated the
operation until no appreciable fresh quantity came into
solution, a condition generally reached after four to five
extract! »ns.

Possibly the yield could be still further increased by
presenting the coal in a very finely divided state to the
solvent, and also by applying the principle of the
Soxhlet method, namely continuously supplying fresh
solvent to the material under treatment. The attempt
to do this, however, has had to be abandoned for the
present, owing to the difficulty of constructing such an
apparatus to stand pressures of 50 to 60 atmospheres.

Description of the Experiments.—The apparatus used
consisted of a steel bomb, 12 cm. wide inside,
62 cm. long, and of metal 2 cm. thick, provided
with a pressure gauge and tested to 200 atmospheres.
Inside the bomb was suspended a wire gauze basket
(9 sq. mm. mesh), so as to be 1 to cm. from the sides